Most of the primary copper produced in the world is derived from sulfidic copper ores, such as chalcopyrite (CuFeS.sub.2). In the conventional processing of sulfidic copper ores to obtain copper, the ore is smelted in air, thereby producing large amounts of sulfur dioxide from the bound sulfur in the ore. In order to meet air pollution control requirements, most of the sulfur dioxide must be recovered in some manner.
Sulfur dioxide emission control processes are expensive and increase the cost of producing copper significantly. In addition, disposal of the recovered sulfur dioxide frequently presents a problem because of its harmful effect upon the environment. Only when the sulfur dioxide is converted into elemental sulfur is the disposal or stockpiling completely satisfactory from the pollution abatement standpoint.
Processes are available for reducing sulfur dioxide to elemental sulfur, including the reaction of the sulfur dioxide with natural gas, carbon monoxide, coal or coke, hydrogen sulfide, carbon disulfide or similar reducing agents. These processes however, are generally considered to be too costly since they are "add-on" processes, i.e. they are not part of the process employed for recovery of copper from the sulfide ore, but rather are added on to the end of the process to dispose of the sulfur dioxide after it has been generated.
Processes for treating sulfidic copper ores have been proposed in which elemental sulfur is formed as part of the decomposition of the ore. In the more significant of these processes, the elemental sulfur is formed by reaction of the ore with concentrated sulfuric acid, aqueous sulfuric acid in the presence of air or oxygen, nitric acid, aqueous ferric chloride, aqueous ferric sulfate, elemental chlorine, etc. All of these processes have been thoroughly investigated, but none of them has achieved commercial success becuase of their expensive nature.
Most sulfidic copper ores also contain significant amounts of iron which, in the recovery of copper, can create a potential pollution hazard. For example, in some processes the iron is disposed of as ferrous or ferric sulfate, basic ferric sulfate or ferric chloride, and dumping of such iron compounds can readily lead to pollution of both the ground and water.